Near-Infrared Photoluminescence Responses of Single-Walled Carbon Nanotubes Induced by Biomolecules Detected on a Microbead Surface

We demonstrate near-infrared (NIR) photoluminescence (PL) microscopy of single-walled carbon nanotubes (SWNTs) attached to individual micrometer-sized bead surfaces. Monodispersed SWNTs wrapped with sodium carboxymethyl cellulose (CMC-SWNT hybrids) were attached to the microbead surfaces, and the functionalized beads were immobilized on a coverslip. It is known that NIR PL spectra of the SWNTs were quenched upon exposure to potassium permanganate (KMnO₄) and recovered by the addition of papain molecules by macroscopic PL spectroscopy. In this study, the PL responses were successfully detected on a microbead surface by NIR microscopy. In particular, PL responses of (6, 5) chirality SWNTs were selectively detected by using two bandpass filters. When KMnO₄ (final concentration: 1 μM) was dropped onto the coverslip surface, the PL intensity of the SWNTs decreased to approximately 20% of the initial intensity without KMnO₄. The average PL intensity of each bead was estimated from the microscopy images. When a papain solution (final concentration: 5 mg/mL) was added to the same sample, the PL intensity recovered to 60-70% of the initial intensity. A similar recovery was observed with papain solutions preheated at 60 and 100 °C. The microscopic observation of the beads was performed sequentially. Although the PL responses induced by papain molecules have been reported in previous studies using macroscopic PL spectroscopy, this is the first report to demonstrate responses on a single-bead surface. Our results hold significance for investigating enzyme reactions using SWNT PL responses at the nano and micro levels.